Plasma processing is widely used in the semiconductor industry for deposition, etching, resist removal, and related processing of semiconductor wafers and other substrates. Plasma sources (e.g., microwave, ECR, inductive, etc.) are often used for plasma processing to produce high density plasma and reactive species for processing substrates. RF plasma sources can be used in modern plasma etch applications because of the ability of the RF plasma source to generate a process chemistry out of feed gases, and to provide an isotropic or highly anisotropic etch at high or low etch rates. Plasma processing tools can be able to sustain a stable plasma in very different gases and under very different conditions (gas flow, gas pressure, etc.) and can provide for independent control of plasma density, plasma profile and ion energy.
Requirements for process equivalence across, for instance, an entire semiconductor wafer, from wafer to wafer in an etch chamber (and/or etch head), from etch chamber to etch chamber across a tool, from etch tool to etch tool across a fabrication facility, or for every fabrication facility across the world can lead to strict requirements for plasma process uniformity in the processing of semiconductor wafers and other substrates. As a result, many plasma processing tools have process controls to compensate for non-uniformities in gas flow, plasma density, and other plasma process parameters. To meet stringent processing requirements for radial process uniformity, some manufacturers employ a variety of methods and systems, such as multiple-coil inductively couple plasma (ICP) sources or multi-zone capacitively coupled plasma (CCP) sources for plasma radial profile control, electrostatic chucks with multiple radial temperature zones, multiple gas injections, etc.
To achieve uniformity in azimuthal direction, some manufacturers design and build plasma chambers, plasma generating parts (e.g., coils, electrodes), electrostatic chucks (ESCs), gas injections and gas exhausts, focus rings, and other components as symmetric as possible to match the symmetry of the semiconductor wafer or other substrate. For this reason, some powered elements (e.g., capacitors) are screened from the antenna and the plasma by placing them outside of the main RF cage in additional enclosures. Wafer placement is also controlled with high precision. Moreover, the magnetic field of the Earth can also affect azimuthal uniformity depending on the location of the tool. For instance, in some cases, the magnetic field of the Earth can affect azimuthal process uniformity for different tools in the same room if the tools face different directions. Some manufacturers wrap champers with magnetic shields to reduce these effects.
In many cases, higher uniformity requirements lead to higher costs of the plasma processing tool. For example, a 1% non-uniformity requirement can require much more design efforts, additional controlling elements, more precise manufacturing of the parts and their assembly, etc. than a 3% non-uniformity requirement. However, imperfection can occur in the design, manufacturing and/or assembly of the process tools. Imperfections in the design can lead to systematic non-uniformities. Imperfections in a manufactured part can lead to both systematic and random (e.g., one tool to another) non-uniformities. Imperfections in assembly can lead to random non-uniformities. When a tool is assembled and all parts are placed in their positions, these non-uniformities are combined together and in the worst case they add to each other rather than compensate each other.
Most of the resulting radial non-uniformity can be compensated by tuning the process using available control “knobs” (powers, temperatures of ESC zones, process time, etc.), but there is no control “knob” for tuning the process in azimuthal direction. So if azimuthal non-uniformity or azimuthal head-to-head mismatch exceeds an acceptable limit, then the only way to fix it, is to identify and replace parts with the largest contribution to azimuthal non-uniformity This procedure can be expensive and time consuming, leading to increased final production cost and increased price of the tool. In some cases, the parts that are replaced may not be bad parts at all and in combination with different other parts the overall non-uniformity of the tool could be below that limit.
The number of elements in and around a plasma processing chamber affecting plasma and process uniformity is quite large. These parts can include, for instance, a gas injection and gas outlet, coils, a pedestal with bias electrode, and an electrostatic chuck (ESC), an RF cage, chamber walls and the door for wafer transfer, ESC feed structure, magnetic field from earth and from elements of the structure—frame, screws, turbo pump, cooling fans, and other elements. Contribution to asymmetry from some of these parts can be made very small. However components such as ESC and coils cannot be made absolutely symmetric. For instance, the ESC can have a complicated structure with a number of electrodes, heating elements and cooling channels, bonding between different layers, etc. Coils can have leads and transitions from one turn to another. One may decrease the magnitude of non-uniformity associated with coil leads, by increasing number of leads and spreading the leads around (e.g., 3 one-turn coils oriented 120 degrees relative to each other instead of one 3 turn coil, or two 1.5 turn coils having leads on the opposite sides, etc.). However, this may lead to additional costs and may not provide for improved azimuthal uniformity.
One approach for addressing azimuthal non-uniformity is to provide for the tilting of the coil as a whole, with respect to the wafer (and/or the pedestal). However, this solution may not be practical because it can cause a very strong effect and cannot be used for fine tuning when the azimuthal non-uniformity is already small.